Bug may expose encrypted e-mail

A problem related to a widely used open-source cryptography technology could let miscreants tamper with digitally signed and encrypted e-mails.

The problem lies in how certain e-mail applications display messages signed using the GNU Privacy Guard, also known as GnuPG and GPG, the GnuPG group said in a security alert Tuesday. It may not be possible to identify which part of a message is actually signed if GPG is not used correctly, it said.

"It is possible to insert additional text before or after a signed, or signed and encrypted, OpenPGP message and make the user believe that this additional text is also covered by the signature," according to the alert.

Several open-source e-mail clients are affected by this latest issue, according to security company Core Security Technologies, which discovered the issue. The list of affected applications includes KDE's KMail, Novell's Evolution, Sylpheed, Mutt and GnuMail.org, according to Core. Enigmail, an extension to the Mozilla mail clients, is also vulnerable, the security research company said.

"It is important to note that this is not a cryptographic problem. It affects how information is presented to the user and how third-party applications interact with GnuPG," Core said in an alert.

In addition to adding content to seemingly secure e-mails, attackers can exploit the problem to bypass content-filtering defenses such as antispam mechanisms, Core said.

GnuPG is a free replacement for the Pretty Good Privacy cryptographic technology. An e-mail that uses OpenPGP cryptography can be made up of multiple sections, not all of which need to be signed or encrypted. E-mail programs that do not correctly interpret the message could indicate that a message is fully secure when, in fact, it is not.

"You see the pretty icon telling you that the whole message is encrypted and signed, whereas there is a section of it--text, image, binary, whatever--which isn't," Arrigo Triulzi, a SANS Internet Storm Center staffer, wrote on the organization's blog.

The GnuPG group has issued updates to prevent tampering with signed or encrypted messages, but it notes that individual e-mail applications might need updating as well, to correctly display signed messages after applying the GPG update.

"After applying one of these patches, some vulnerable applications may fail to handle certain messages," the GnuPG alert states. "Fixing the application is required, as there is no way for GnuPG to do it."

Core also published a work-around to help users detect and prevent exploitation. If a signed message looks suspicious, the validity of the signature can be verified by manually invoking GnuPG from the command line and adding the special option "--status-fd" to gain extra information, Core suggested.